The present invention relates generally to gas turbine engines, and, more specifically, to wide chord fan blades therein.
A turbofan gas turbine engine includes a row of fan blades powered by a low pressure turbine (LPT). Air initially enters the engine through the fan, and an inner portion thereof enters a compressor which pressurizes the air for mixing with fuel in a combustor, with the mixture being ignited for generating hot combustion gases that flow downstream through a high pressure turbine (HPT) that extracts energy for powering the compressor. The combustion gases then flow through the LPT which extracts additional energy therefrom for powering the fan. The remaining outer portion of the air flowing through the fan is discharged from the engine for producing thrust to power an aircraft in flight.
A fan blade includes a dovetail at its radially inner end which is trapped in a complementary dovetail slot in the perimeter of a rotor disk. An airfoil is attached to the dovetail by a structural shank. Platforms may be joined integrally with the blade or separately attached between adjacent blades for providing a radially inner flowpath boundary for the fan air, with the platform being radially located atop the shank at a radially inner root of the airfoil.
The airfoil extends radially outwardly to an opposite tip, and has a forward or leading edge and an axially opposite aft or trailing edge collectively defining the perimeter of the airfoil. The airfoil has a generally concave, pressure first side and a circumferentially opposite, generally convex, suction second side. The airfoil has a span or longitudinal axis extending in the radial direction from the centerline of the rotor disk to which it is attached, and various chords extending generally axially between the leading and trailing edges. The airfoil typically twists from its root to its tip for maximizing aerodynamic performance.
A wide chord fan blade has a relatively low aspect ratio which is its span to chord ratio, and is relatively heavy when formed as a solid metal part. Weight reduction is typically obtained by using high strength superalloy materials such as those including titanium. However, as engines grow larger in size, the corresponding fan blades increase in size and weight, and increase the difficulty in achieving a suitable life therefor under the high centrifugal loads generated during operation.
In separate developments, all-composite fan blades have been designed for reducing weight while providing acceptable performance in the gas turbine engine environment. A typical composite blade includes several layers of structural fibers, such as graphite, embedded in a suitable matrix, such as epoxy, for tailoring blade strength in a lightweight structure. Composite blades require a complex manufacturing process and are expensive to produce.
Hybrid blades are also being developed which are primarily metal, such as titanium, having suitable pockets therein for reducing weight, with the pockets being filled with a suitable elastomeric filler material for completing the required aerodynamic profile of the airfoil. The pockets are defined by corresponding integral metal ribs which provide metal across the full thickness of the airfoil for maximizing the remaining stiffness and bending moment of inertia of the airfoil.
However, the weight-reducing pockets necessarily interrupt the structural continuity of the airfoil, with the exposed edges of the pockets creating structural discontinuities in the airfoil surface adjoining the filler material. The ribs are therefore subject to local stress concentration during operation.
During operation, the fan blades rotate and are subject to centrifugal loads which are carried by the metal portion of the airfoils including the ribs, with the filler merely providing dead weight which in turn is carried by the metal portion of the airfoil. The airfoil is subject to vibratory bending and torsion which in turn increases the loads and resulting stress carried by the metal airfoil including the ribs thereof. And, the fan blade is subject to foreign object damage (FOD) due to impact from a bird strike for example. A bird strike subjects the blade to additional shock loading which further increases the stress of the metal airfoil including the ribs.
In order to ensure a strong bond between the filler and the underlying metal of the airfoil, a primer coat may be applied in the pockets prior to filling with the filler material. When the filler is cured and bonded to the metal airfoil, its outer surface is directly exposed to the ambient air and is coextensive or flush with the remaining metal border of the airfoil for defining the corresponding aerodynamically configured side of the airfoil.
However, the underlying primer is exposed along the edge interface between the filler and the metal border and is therefore subject to degradation due to humidity, chemical solvents, and handling damage. Degradation of the primer can then lead to delamination of the filler from the metal airfoil, and a corresponding reduction in useful life of the fan blade.
Accordingly, it is desired to provide an improved hybrid fan blade having reduced stress concentrations and reduced exposure of the filler-metal interface.
A fan blade includes a metal airfoil having a pocket disposed in a first side thereof, with the pocket including a filler bonded thereto. The pocket includes a plurality of cells separated by corresponding ribs which are submerged in the filler.